Conventionally, a radiotherapy apparatus includes an X-ray tube for generating an X-ray image and a flat panel detector in order to check a position of a lesion area of a patient. In such a radiotherapy apparatus, there is a known technique in which positioning of various devices is performed with reference to a phantom disposed at the isocenter.                [Patent Document 1] Japanese Unexamined Patent Application Publication No. 2016-221156        
In the technique described above, X-ray imaging is performed to generate an X-ray image depicting a phantom and the position of the flat panel detector is adjusted on the basis of this X-ray image.
However, there is a problem that work efficiency of the position adjustment of the flat panel detector is poor because it is necessary to alternately repeat the X-ray imaging and the movement of the flat panel detector until the flat panel detector is moved to a proper position.
In view of the above-described problem, an object of embodiments of the present invention is to provide such a position adjustment technique for a flat panel detector that work efficiency of position adjustment of a flat panel detector can be improved.